MAMMARY GLAND: SECRETION OF MILK. 315 poured into the canal leading to the internal ear, for the purpose (it would seem) of preventing the entrance of insects. 376. The secretion of Milk is important, not so much to the parent who forms it, as to the offspring for whose nourishment it is destined. It does not seem to carry off from the system any injurious product of its decomposition; for it bears a remarkable analogy to blood in the combination of substances which it contains; nevertheless it is found that, when this secretion is once fully established, it cannot be suddenly checked, without producing considerable disturbance of the general system. The structure of the Mammary gland closely resembles that of the parotid already described (fig. 165). It consists of a number of lobules, or small divisions, closely bound together by fibrous and areolar tissue; to each of these proceeds a branch of the milk-ducts, together with numerous blood-vessels; and the ultimate ramifications of these ducts terminate in a multitude of little follicles, about the size (when distended with milk) of a hole pricked in paper by the point of a very fine pin. 377. The nature of the composition of Milk is made evident by the processes to which we commonly subject it. When it is allowed to stand for some time, its oleaginous part, forming the cream, rises to the top. This is still combined, however, with a certain quantity of albuminous matter, which forms a kind of envelope round each of the oil-globules; but in the process of churning, these envelopes are broken, and the oil-globules run together into a mass, forming butter. In ordinary butter a certain quantity of albuminous matter remains, which, from its tendency to decomposition, is liable to render the butter rancid; this may be got rid of by melting the butter at the temperature of 180°, when the albumen will fall to the bottom, leaving the butter pure and much less liable to change. In making cheese, we separate the albuminous portion, or casein, by adding an acid which coagulates it. The buttermilk and whey left behind after the separation of the other ingredients, contain a considerable quantity of sugar, and some saline matter. The proportion of these ingredients varies in different animals; and also in the same animal, according to the substances upon which it is fed, and the quantity of exercise it takes. The amount of casein seems to be greatest in the milk of the Cow, Goat, and Sheep; that of oleaginous matter in the 316 GENERAL REVIEW OF NUTRITIVE OPERATIONS. milk of the Human female; and that of sugar in the milk of the Mare. The milk of the Cow, if a portion of its casein were removed, would resemble Human milk more nearly than any other; and it is therefore best for the nourishment of Infants, when the latter cannot be obtained. The important influence of Mental emotion on this secretion has already been noticed (§ 353); and many more instances might be related, were not the ordinary facts in regard to it generally known. CHAPTER VIII. GENERAL REVIEW OF THE NUTRITIVE OPERATIONS-FORMATION General Review of the Nutritive Operations. 378. In the preceding Chapters (III. to v.) those processes have been described, by which the alimentary materials that constitute the raw material of the tissues, are converted into a fluid adapted for the Nutrition of the body; and we then (CHAPS. VI. and VII.) considered those functions, by which this fluid is kept free from the impurities it acquires during its circulation through the body, and is maintained in the state which alone can adapt it to the purposes it is destined to fulfil. These purposes may be regarded as fourfold. In the first place, the Blood is destined to supply the materials of the fabric of the body; which, as it is continually undergoing decay ($ 68), requires the means of as constant a renovation. Secondly, the Blood (in most animals at least) serves to convey to the tissues the supply of oxygen which is required by them, especially by the muscular and nervous tissues,-as a necessary stimulus to the performance of their functions. Thirdly, the Blood furnishes to the secreting organs the materials for the elaboration of the various fluids, which have special purposes to serve in the Animal economy, such, for instance, as the Saliva, Gastric juice, Milk, &c. And lastly, the Blood takes up, in the course of its circulation, the products of the waste or decomposition of the various tissues, which it conveys to the several organs,-the Lungs, Liver, FORMATION OF THE TISSUES. 317 Kidneys, &c.,-destined to throw them off by Excretion. The greater number of these processes have already been treated of in more or less detail. Those included under the first head were considered, in a general form, in Chap. I. of this Treatise. Those which are comprehended under the second head have been dwelt-on in Chaps. v. and vI.; and will be again noticed, when the actions of the Nervous and Muscular tissues are described. And the varied actions which are included under the third and fourth classes, have been discussed in the two Chapters which precede the present one. We have now to enter, in more detail, into the mode in which the circulating fluid is applied to the Nutrition and Formation of the Tissues. Formation of the Tissues. 379. There is sufficient reason to believe that every living being is developed from a germ; no organized structure being able to take its origin (as some have supposed) in a chance combination of inorganic elements. All the facts relating to the production of Fungi and Animalcules, which have been imagined to favour this doctrine, may be satisfactorily explained in other ways (VEGET. PHYS. § 779; ZOOL. § 1213). Now the first structure developed from this germ, in the Animal as in the Plant, is a simple cell; and the entire fabric subsequently formed, however complex and various in structure, may be considered as having had its origin in this cell. The cells of Animals, like those of Plants, multiply by the development of new cells within them; each of these becomes in its turn the parent of others; and thus, by a continuance of the same process, a mass consisting of any number may be produced from a single one. It is in this manner that the first development of the Animal embryo takes place, as will be shown hereafter (Chap. xv.). A globular mass, containing a large number of cells, is formed before any diversity of parts shows itself; and it is by the subsequent development, from this mass, of different sets of cells,-of which some are changed into cartilage, others into nerve, others into muscle, others into vessels, and so on,-that the several parts of the body are ultimately formed. 380. This process of differentiation is carried to very different degrees in the development of the several classes of 318 DIFFERENTIATION OF STRUCTURE, animals; for in some it is checked so early, that scarcely any distinction either of organs or of tissues ever shows itself; whilst in others it continues during a large proportion of the earlier period of life. It has no relation to growth, or simple increase of size; for this may take place by the multiplication of similar parts, giving rise to that "vegetative repetition" which is so characteristic of the lower tribes of Animals, and which gives to many of them so strong a resemblance in general aspect to Plants; whilst, on the other hand, the developmental process by which higher forms of structure are evolved, sometimes takes place without any increase at all. It is in its degree of such differentiation, that what is called the lower or the higher organization of any living being essentially consists; for whilst in the simplest forms of Animal structure every part is similar to every other, so that all the functions of life are performed in common by each, we find in Man (whose body may be regarded as presenting the highest type or example of this differentiating process) that no two parts are precisely similar, except those on the opposite sides of the body. This fact is occasionally manifested in a very singular manner, in the symmetry of disease; certain morbid poisons (as those of gout, and of several affections of the skin), which have a tendency to single out particular spots of the tissues whose nutrition they disturb, exhibiting their action in those parts of the two lateral halves of the body which precisely correspond with each other. 381. Now in the lowest grades of Animal structure, we find that the several tissues of the body can themselves appropriate from the products of digestion the nutrient materials they respectively require; so that, for their growth and maintenance, it is sufficient that these products should be carried into their neighbourhood by extensions of the digestive cavity (§ 296). But in all the more highly-organized animals, it appears requisite that the nutrient material should pass through an intermediate stage of preparation, which is termed assimilation (or making-like); and this is effected by their introduction into the current of the circulation, and their mixture with the pre-existing blood, which, in virtue of its own vital powers, exerts upon them a converting action, which prepares them for being appropriated by the solid tissues. 382. When once the several forms of tissue have been MUTUAL DEPENDENCE OF PARTS OF ORGANISM. 319 developed, their nutrition is kept up by the supply of their respective materials which they derive from the blood. Each tissue draws from the circulating current that which it requires; and it is one of the most wonderful proofs of the skill with which the entire fabric has been planned and constructed, that the composition of the blood should be maintained at a nearly uniform standard, in spite of the continual change which is thus taking place in its actual components. It has been justly remarked, that each part of the body, by taking from the blood the peculiar substances which it needs for its own nutrition, does thereby act as an excretory organ, inasmuch as it removes from the blood that which, if retained in it, would be injurious to the nutrition of the body generally. 383. Hence it seems that such a mutual dependence must exist among the several parts and organs of the body, as causes the evolution of one to supply the conditions requisite for the production of another; and this view is borne out by a great number of phenomena of very familiar occurrence, which show that a periodical change in one set of organs governs changes in others which at first sight might seem to have no relation to them. Thus the plumage of Birds, at the commencement of the breeding season, becomes (especially in the male) more highly coloured, besides being augmented by the growth of new feathers; but when the generative organs pass into their condition of periodical inactivity, the plumage begins at once to assume a paler and more sombre hue, and many of the feathers are usually cast, their nutrition being no longer kept up. So, again, it is no uncommon occurrence among Birds, for the female, after ceasing to lay, to assume the plumage of the male, and even to acquire other characteristic parts, as "spurs" in the fowl tribe. That, in these and similar instances, the development of organs is immediately determined by the presence or absence in the blood of the appropriate pabulum for the parts in question, and that its existence depends upon changes taking place in other parts, has been rendered still more probable by the results of experiments, which show that if the ordinary changes in one set of organs be prevented by their removal, those usually taking place in the others do not occur. 384. Though all the tissues derive the materials of their development from the blood which circulates in the vessels, |